Ink jet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that may be utilized to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand ink jet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal ink jet (TIJ)) and piezoelectric transducers. In another process known as continuous ink jet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught and recycled to an ink sump. Ink jet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to short run printing and industrial labeling.
Ink compositions used in ink jet printers can be classified as either pigment-based in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based in which the colorant exists as a fully solvated dye species that consists of one or more dye molecules. A sub-class of dye-based inks may contain polymeric colorants in which a chromophore is covalently bound to a polymer structure. Pigment-based inks are often preferred over dye-based inks because they possess better resistance to light and gas, especially ozone, as compared to printed images with dye-based inks.
Pigment-based inks have a number of drawbacks. Great lengths must be undertaken to reduce a pigment to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image. Pigment inks are also difficult to jet through ink jet print heads having small nozzle diameters, for example, less than 25 μm and numerous approaches have been taken to formulate pigment based inks for piezo and thermal ink jet print heads.
Self-dispersed carbon black pigment-based ink compositions lacking a film-forming polymer binder offer high optical density on untreated bond papers that approach electrophotographic-printing quality, with values of about 1.4. The colorant, however, is readily re-dispersed by wet rub abrasion, resulting in undesirably low durability. Polymer-dispersed carbon black pigment ink compositions of the art offer excellent water-fastness, wet rub durability, and dry rub abrasion on all substrates, but optical density suffers on plain papers, where the colorant apparently wicks along the cellulose fibers into the interior of the paper, leading to grayish appearing printed text.
Continuous ink jet printing has related needs for improved ink compositions. High-speed continuous ink jet printing is used in commercial market applications and generally involves printing variable information for transactional documents such as invoices and credit card billing statements, and also scratch-off lottery tickets. Variable-data imprinting sub-systems, consisting of a printhead, control electronics, an ink reservoir, an ink pump and an ink delivery system, can be added to an existing high-speed press system for black text printing in labeling or mailing applications. Commonly used dye-based inks can provide adequate optical density on the normal mix of paper substrates, such as plain bond papers, surface-treated papers, or coated and calendared business gloss papers or heavy-stock covers. Dye-based inks, however, suffer poor waterfastness on all substrates, and low durability on glossy papers against wet rub abrasion that can render text and universal packaging code information illegible.
Water-fast polymeric colorants made from polyethyleneimines are known in the art of inks and ink jet printing such as those described in U.S. Pat. Nos. 4,664,708 and 4,375,357. U.S. Pat. No. 5,534,052 discloses aqueous inkjet inks containing polymeric colorants comprised of polyamidoamines bonded with a reactive dye through an amino group. U.S. Pat. No. 4,471,079 discloses aqueous inks containing emulsion particles of a polymeric dyestuff prepared by reacting an anionic dye or a reactive dye with a water-insoluble polymer containing amino groups. U.S. Pat. No. 5,250,107 discloses aqueous inkjet inks containing an ammonium zirconium polymer and a reactive dye which, upon dehydration, crosslink to form a waterfast dye complex. U.S. Pat. Appl. 2004/0147631 discloses aqueous inkjet inks containing a polymeric colorant made from a dye linked into a polymer skeleton. The copolymer colorant consists of a hydrophobic monomer and a hydrophilic nonionic monomer.
U.S. Pat. No. 5,879,439 discloses aqueous inkjet inks containing both a pigment and a polymeric dye where the polymeric dye has an average molecular weight of 5,000 to 15,000 and contains carboxylate groups which are preferred from a standpoint of convenience of production. U.S. Pat. No. 4,181,185 discloses a method of printing polymeric colorants onto a paper stock having therein polyvalent metal cations resulting in water fast images. U.S. Pat. No. 5,030,697 discloses polymeric colorants where the polymer backbone is a copolymer of acrylamide monomers and maleic anhydride. These colorants are further functionalized to enable covalent bonding to biological materials. EP 1 489 144 A1 discloses styrene maleic anhydride based copolymeric dyes that are designed for ink jet inks and have good fabric and skin washability. However, these polymeric dyes are not designed to be water-fast on an ink jet receiver.
Polymeric colorants based on copolymers of styrene and maleic anydride comprising a dye, a pigment and a dispersant functionality covalently attached to the polymer backbone, are disclosed in United States Patent Publication Nos. 2006/0089421 and 2006/0089422.
There remains a need to provide a polymeric colorant that can be utilized in an ink composition that results in high density and good water-fastness when printed onto a wide range of receivers including uncoated or plain paper. In addition, there remains a need to provide a polymeric colorant which can be utilized in an ink composition for ink jet printing which is stable over long periods of time and does not adversely affect the performance of an ink jet nozzle.